This is slightly off-topic, but, I hope, sufficiently Mac-focussed to have a place here.
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dan8394Aug 25 '11 at 16:24

A good technical nugget lies at the heart of this question. "Are HTML formatted emails inherently scaleable in arbitrarily sized Lion mail.app windows or is the app interpreting/correcting these messages to provide superior scaling and display of messages?"
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bmike♦Aug 25 '11 at 17:39

4 Answers
4

HTML emails from companies are usually made using an online HTML builder, such as xact target, Dot mailer, Mail Chimp and the like.

The templates use HTML to create the emails. In the HTML there is code which will render differently in Mail, and outlook, Hotmail, yahoo mail, etc. The suppliers of email software will spend significant amounts of time making sure teh same version is seen in the same way by all email recipients

The HTML code in a lot of HTML email also defines whether you are using a desktop app, web mail, or even mobile.

The reason your emails fit onto your browser is down to the flood of people using mobile email, on iPhones, iPads, Android devices, etc. The email is built in columns, usually no more that 2. These columns can then be narrow or wide depending on whether the recipient is using a mobile device or a desktop email client.

Next time you get a HTML email have a look at it on your desktop email, and then compare it with how it looks on a mobile.

The hardest mobile to support is Blackberry, as most models do not support HTML, instead they often render a plain text version.

With the right usage of HTML and CSS you can craft HTML emails in a way that they expand to fill whatever size window/message pane they're displayed in. Most websites do this to some degree (open a site and change the size of your browser window for an effective demonstration).

Some further information from a blog that happened to be the first Google hit. No guarantee of it's quality, but it looks to be a decent first start if you want more details.

HTML-formatted e-mail is actually a hairy subject that goes way beyond the scope of Apple-related stuff. Considering all the web-based mail clients and desktop mail clients, each of which has its own rendering quirks, creating HTML e-mail that looks more or less the same in all of them is a serious challenge. MS Outlook is an especially notorious offender in terms of wonky rendering.

With a relatively simple design, it's easy to craft an e-mail that will fill the client's window. When you start getting into multiple columns and such, it gets trickier.

There are no current industry standards for HTML e-mail (other than MIME encoding, that sort of thing); there are some generally understood best practices for producing HTML e-mail that will turn out OK in most clients, which have been worked out through trial and error.